Recent infrared (IR) video camera systems employing IR sensitive elements, e.g., a Charged Coupled Device (CCD) focal plane array, are generally provided with a plurality of detachably mounted imaging lens units. This allows a user to interchangeably mount one lens unit at a time to the body of the video camera system, based on the object scene to be captured. Each imaging lens unit is configured to image at least a portion of the object scene onto the focal plane array, depending upon the field of view of the imaging lens unit. For example, a telephoto-type imaging lens unit will generally provide a relatively narrow viewing field, while other types of imaging lens units will provide a wide angle or mid-range field of view.
Such infrared video camera systems may also include a plurality of interchangeable optical filter units which are configured to filter the energy of the object scene prior to capture by the IR sensitive element. Such optical filter units may include spectrally narrowing bandpass filters, e.g., for preventing non-IR wavelengths from reaching the IR sensitive element, or for simply narrowing the spectral range of viewing, neutral density filters for attenuating the luminous energy reaching the IR sensitive element, and the like.
In infrared video camera systems of this type, both the individual imaging lens units and the individual optical filter units are identified to a microprocessor contained within the body of the camera, and a characteristic data set, stored in a memory device in the camera body, is made available to characterize or tailor the response of the IR sensitive sensor element to an object scene image. A set of characteristic data is stored in the camera memory for each type of imaging lens unit and optical filter unit which may be installed on the infrared video camera system. A typical characteristic data set may include a bias voltage for the IR sensitive element, a sensor integration time, an equalization table of pixel by pixel gain and/or offset data, and absolute temperature calibration data. In each case, the characteristic data set stored in the camera memory is predetermined to modify the camera system for operation with a corresponding imaging lens unit or optical filter unit.
Imaging lens and filter unit identifiers have heretofore utilized Hall-effect sensors mounted to the camera body, and one or more permanent magnets mounted onto the imaging lens unit housing and optical filter unit housing, to identify the imaging lens unit and optical filter unit to the microprocessor of the camera system. When the imaging lens unit and optical filter unit are attached to the camera body, the permanent magnets on the imaging lens unit and optical filter unit are aligned with the Hall-effect sensors attached to the camera body. The microprocessor software determines the status of the Hall-effect sensors, identifies the lens and filter configuration of the imaging lens and optical filter units based on the status of the sensors, and selects the appropriate characteristic data set from the camera memory.
A major problem with this approach is that one Hall-effect sensor is required for each binary bit of information. For example, a combination of 16 separate identifiers requires 4 sensors, since 2.sup.4 =16. Further, since the Hall-effect sensors require a relatively large amount of space, and one or more wires must be attached to each Hall-effect sensor, the number of sensors which can be used is limited by the space and wiring constraints of the camera system. Clearly, including more substantial identifying information, such as lens transmission data, serial number or date placed in service, is not practical because of the large number of sensors that would be required.
A further disadvantage associated with the use of Hall-effect sensors is their susceptibility to magnetic fields. Occasionally, when used in or near strong magnetic fields, the output of the Hall-effect sensors may be adversely affected, potentially resulting in the misidentification of an attached imaging lens unit or optical filter unit, and the use of an incorrect characteristic data set for the operation of the camera system.